Abstract : The general problem of electromagnetic coupling from one region to another through an aperture to cavity to aperture system is specialized to the case of electrically small circular apertures in a cylindrical cavity of circular cross section. A simple equivalent circuit for the coupling system is developed. It is found that for certain cavity depths an exceptionally large amount of electromagnetic energy can be transmitted, and that for identical apertures the transmission cross section of the system is independent of the size of the apertures. (Author)